Electromagnetic scattering with interaction between the perfect conducting plate and the 2-D conducting slightly rough surface is investigated. Taking the advantage of s newly developed technique that utilizes the reciprocity theorem, the difficulty in formulating the secondary scattered fields from the composite target is reduced to the evaluation of integral equation involving the polarization currents of the conducting plate and the scattered fields from the slightly rough surface. The polarization currents on the plate and the electromagnetic scattering field are solved by using physical optics approximation and small perturbation method, respectively, and the solution for the composite scattering cross section is obtained and calculated. In particular, the results of backscattering from the composite mode are discussed in detail.